The present invention relates to an apparatus for immediately outputting the response of a synchronous system to an asynchronous event.
A synchronous system is a system whose state changes only at specific (normally equidistant) times. Such a system is, for example, a digital circuit in which the sequential elements (the flipflops) and thus, the circuit containing the sequential elements as well, itself changes its state only during the rising or falling edge of a clock signal, or shortly after it (delayed by the gate delay times). The times at which state changes can occur are referred to in the following text as defined state changing times.
In contrast to this, an asynchronous event is an event that may occur at any time.
Since synchronous systems react on certain occasions with a defined result to events that occur more or less precisely at the defined state changing times, it has been found to be advantageous for the asynchronous events (or more precisely, the signals or signal changes which signal such events) to be synchronized (phased-in). This can be done, for example, by connecting a flipflop downstream from an input connection via which the asynchronous event is fed into the system, with the input of the flipflop and the asynchronous input connection of the synchronous system being connected to one another. Since signals applied to the flipflop input are transferred to the flipflop output only on the rising or falling edge of a clock signal, a synchronous (phase-in) signal is available at the flipflop output.
This makes it possible to ensure that the synchronous system always reacts correctly to asynchronous events.
Generally, however, the synchronous system does not respond immediately to an asynchronous event, but with a greater or lesser delay, since synchronous systems can in fact change their state only at the defined state changing times.
However, in certain cases, it is necessary to react to the occurrence of an asynchronous event immediately.
In order to achieve immediate reaction to asynchronous event in a synchronous system, it is possible for those parts of the system to which asynchronous events are input and which most produce a response to these events to be operated asynchronously. The relevant system parts can be operated asynchronously, for example, by simultaneously using the asynchronous event as the clock signal for these system parts, as a result of which the elements in the relevant system parts switch at the same time that asynchronous events occur. In this manner, it is possible to respond to asynchronous events immediately. However, a problem in this case is that the synchronously operating part of and the asynchronous part of it change their states at different times and must be synchronized with one another in a complex manner.